A conventional electrical power generating system (EPGS) for an aircraft, in one known form, comprises an integrated drive generator including a constant speed drive and a generator. The integrated drive generator receives mechanical power at varying speed from an aircraft engine, and delivers electrical power at a constant frequency. The constant speed drive includes a speed control assembly which receives mechanical input power at varying speeds from the aircraft engine, and which delivers power from its output shaft at a constant speed. The generator comprises a salient pole machine with a rotating field which is excited through an exciter powered by a permanent magnet generator (PMG) through a voltage regulator. Such conventional systems use a generator control unit (GCU) to provide voltage regulation and speed regulation. Specifically, a voltage regulator provides excitation power to an exciter at levels which provide constant system voltage at the point of regulation. A speed controller controls the trimming of a servo valve to maintain constant generator speed, and thus constant frequency.
Prior generator control units used either analog or digital circuits, with the choice being based on factors such as weight, size, cost, and complexity of control logic. In analog GCU systems, both integrated circuits and discrete components are used. The analog input signals are typically combined, and perform their required functions, using analog type controls. Such system products incorporate standard, off-the-shelf components. Implementing a system which has the complexity of a generator control unit with standard product technology requires the use of many hundreds of electrical devices even for a relatively simple application, such as for a single-channel EPGS. Each device adds additional weight to the product, including indirect weight in the form of additional circuit board area and housings required to support the inclusion of each device. Since commercial and military aircraft are the intended end-use of such products, it is desirable to minimize this additional weight. Furthermore, analog circuits tend to be environmentally sensitive. For example, parameter drift results owing to changes in temperature and humidity, as well as age of the devices. Moreover, using analog technology, the control functions cannot be easily modified. Instead, circuit components must be changed, resulting in a custom design for each different application.
Conversely, in digital control systems, all of the analog input signals are converted to digital form, and certain control and protection functions are controlled by a microprocessor and associated software. As such, the control system is inherently more flexible in implementing different control schemes. The microprocessor continuously and sequentially checks for proper system conditions, and performs the programmed sequence of instructions upon proper system commands.
However, the actual flexibility of such a digital system is limited due to the severe constraints on processing time available to the microprocessor, which must perform both control and protection functions. In fact, known GCU systems employ analog control circuitry for implementing the voltage regulator functions. As a result, it is necessary to provide individual circuit components which are solely associated with voltage regulation. Therefore, the designer must "start from scratch" in designing a generator control unit for each new application. This results in each generator control unit being custom-made and therefore more expensive.
In the aforementioned copending application, a digital voltage regulator is disclosed for a generator control unit. The voltage regulator comprises a digital control circuit including a processor having a memory circuit. The processor is responsive to system condition inputs for establishing different parameters of the control signal in accordance with a voltage regulation algorithm. In performing the voltage regulation function, the processor executes multiple sets of instructions corresponding to multiple feedback controls loops. At least seven interrelated feedback control loops are utilized in the digital voltage regulator chip. Such sophisticated feedback control loops require equally sophisticated analog-to-digital conversion techniques, which were previously unnecessary for prior analog systems or simple digital systems.
Known A/D interfacing techniques are ill-suited for use with many complex digital control systems, such as the multiple-loop digital voltage regulator in the GCU. First, an A/D converter having an extremely fast operating rate would be required to keep up with the digital voltage regulator processor. Using today's technology, an A/D converter having such a fast conversion rate would be prohibitively expensive. Second, if a slower A/D converter were utilized, the feedback control loops would not be operating on the most up-to-date information, and therefore would become unstable. Third, most A/D conversion techniques are limited to specific controllers in particular applications. However, the digital voltage regulator and GCU of the present invention are designed to be employed in a wide variety of applications without hardware redesign. Hence, compatibility with A/D converters having different operating speeds is necessary to maintain design flexibility.
A need, therefore, exists for an improved multiplexing analog-to-digital conversion technique adapted for acquiring data derived from a plurality of analog input signals, and having the flexibility to utilize A/D converters with various conversion rates.